The adrenal medulla is less of a 'real' endocrine organ like the others in the endocrine system and much more an extension of the sympathetic nervous system. In fact, its chromaffin cells are modified neurons by descent and secrete adrenalin and some noradrenalin upon stimulation by sympathetic preganglionic fibres, effectively turning the medulla into a sort of 'endocrine ganglion' with the whole cardiovascular system as its 'postganglionic fibres'.

Adrenalin in the circulation seems to have pretty much the same effect as all the adrenergic neurons of the sympathetic nervous system (which innervates all blood vessels, organs etc.): arteriole constriction, cardiac output increase, breath rate increase, pupil dilation, glucagon secretion and insulin inhibition, stimulation of glycolysis and glycogenolysis etc.

So what is the purpose of the adrenal medulla, if all of the effects which the endocrine adrenalin produces are essentially the same as those produced by the sympathetic mass response?

2 Answers
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I would argue that the adrenal medulla is a real endocrine tissue for the following reasons. First and foremost, it fits the definition of an endocrine tissue, which is to produce and secrete hormones in a regulated manner. You've already highlighted the physiological effects of those hormones, and I would ad dopamine as the other major secretory hormone/neurotransmitter. Chromaffin cells are dervived from neurons, but the presence of a regulated secretory pathway is a hallmark of endocrine tissues. @Armatus answer of plasticity is on the mark, although I think a term that more readily describes the system's ability react over a greater time scale would be impedence matching. Another differentiating characteristic is that the adrenal medulla is capable of sustained synthess and secretion in response to stimuli, whereas I don't think this is typical of most neurons which transmit a message only briefly.

I agree with you - of course it is an endocrine tissue, that's not the question here. The question is rather about its purpose when the wide-spread effect of adrenaline produced by it is also achieved by the sympathetic mass response :) I agree that plasticity was a bad choice of word. Your last point is correct and complements my answer. Such upregulation is possible for neurons too as far as my teaching has been (persistent firing), but it would be against the purpose of the sympathetic nervous system (quick response to stress) to use it in this kind of way.
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ArmatusJan 6 '13 at 11:03

I think we're agreed then: the adrenal medulla is a complementary organ to the mass response of the SNS. It is also more than that, acting as an endocrine tissue, which does take input from other endocrine tissues to modulate any sustained responses to stress.
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leonardoJan 6 '13 at 18:04

I have found the answer to the purpose of the adrenal medulla to be flexibility. Just like regulation for example of gene expression or metabolism occurs on many levels with different speeds and permanences of their effects, signalling needs to be available both quick and short-lived, and slower but with more long-lasting effects. The adrenal medulla provides exactly that.

Sympathetic adrenergic neurons are quick, but as quickly as they release their neurotransmitter, the effect is only of short duration and adrenalin is quickly cleared from the target area. Rather than giving off short bursts of adrenalin, the medulla maintains a constant systemic level of it and in response to stimulation raises or lowers that level. This response takes more time as blood circulates much slower than neurons respond, but its effects last longer.